Polyketides are a large class of natural products that includes many important antibiotics and immunosuppressants such as erythromycins, tetracyclines, and rapamycins. Their synthesis proceeds by an ordered condensation of acyl esters to generate carbon chains of varying length and substitution pattern that are later converted to mature polyketides. This process has long been recognized as resembling fatty acid biosynthesis, but with important differences. Unlike a fatty acid synthase, a typical polyketide synthase is programmed to make many choices during carbon chain assembly: For example, the choice of "starter" and "extender" units, which are often selected from acetate, propionate or butyrate residues in a defined sequence. The choice of using a full cycle of reduction-dehydration-reduction after some condensation steps, omitting it completely, or using one of two incomplete cycles (reduction alone or reduction followed by dehydration), which determines the pattern of keto or hydroxyl groups and the degree of saturation at different points in the chain is additionally programed. Finally the choice of stereochemistry for the substituents at many of the carbon atoms is programmed by the polyketide synthase.
Because of the commercial significance of Streptomyces, a great amount of effort has been expended in the study of Streptomyces genetics. Consequently much is known about Streptomyces and several cloning vectors exist for transformations of the organism.
Although many polyketides have been identified, there remains the need to obtain novel polyketide structures with enhanced properties. Current methods of obtaining such molecules include screening of natural isolates and chemical modification of existing polyketides, both of which are costly and time consuming. Current screening methods are based on gross properties of the molecule, i.e. antibacterial, antifungal activity, etc., and both a priori knowledge of the structure of the molecules obtained or predetermination of enhanced properties are virtually impossible. Chemical modification of preexisting structures has been successfully employed, but it still suffers from practical limitations to the type of compounds obtainable, largely connected to the poor yield of multistep syntheses and available chemistry to effect modifications. The following modifications are extremely difficult or inefficient at the present time: change of the stereochemistry of the side chains in the completed polyketide; change of the length of the polyketide by removal or addition of carbon units from the interior of the acyl chain; and dehydroxylation at unique positions in the acyl chain. Accordingly, there exists the need to obtain molecules wherein such changes can be specified and performed and would represent an improvement in the technology to produce altered polyketide molecules with predicted structure.